It is a well known technique in the field of usual photography and diffusion transfer photography to provide a temporary barrier layer between layers of a photographic element into which a processing composition permeates.
In the diffusion transfer photography process, in particular, it has been a practice to use a neutralization layer in order to, for example, cease the development or stabilize the image. For preventing too early neutralization of the developing solution and the decrease in maximum density resulting therefrom which may cause by the use of the neutralization layer, it is also widely known to provide a temporary barrier layer as a mechanism for controlling the neutralization timing (i.e., as a timing layer).
In general, the reaction for developing silver halide photographs proceeds quickly at a high temperature but slowly at a low temperature. When the diffusion transfer process is to be applied to instant photography wherein the development is performed not under temperature control but at various temperatures different from the conventional photography, it is therefore highly important that photographic elements are capable of compensating temperature so that the developing solution is neutralized within a short period of time at a high temperature at which the development proceeds quickly but the neutralization is slowly carried out at a low temperature at which the development proceeds slowly, thus giving good images regardless of changes in the development temperature.
As examples of these timing layers capable of compensating temperature, U.S. Pat. No. 3,856,522 proposes to use butyl acrylate/diacetone acrylamide/styrene/methyl methacrylate copolymers; U.S. Pat. No. 4,061,496 proposes to use acrylonitrile/vinylidene chloride/acrylic acid copolymers; and JP-A-53-7266 discloses to use styrene/butyl acrylate/acrylic acid copolymers (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Although the timing layers described in these patents are excellent in the capability of compensating temperature of the neutralization reaction of developing solutions, they suffer from various problems. One of these problems resides in that since these timing layers are made of latex polymer materials, they should be dried at a high temperature under strictly controlled conditions (temperature, humidity, time, etc.), after the application, to give good coating films. Accordingly, complicated application/drying apparatuses are needed, which results in an increase in the cost in many cases. Moreover, the timing layer thus applied frequently undergoes aggregation which lowers the transparency of the film. Furthermore, there arise other troubles such that the timing layer frequently has only a low film strength, the timing layer is frequently peeled off due to poor adhesion thereof to the lower layer, etc.
To solve these problems, attempts have been made to form timing layers by applying polymer solutions in organic solvents. According to JP-A-56-97346, for example, a vinylidene/methyl acrylate/itaconic acid copolymer latex is once powdered and then dissolved in an organic solvent followed by application. Further, U.S. Pat. No. 4,440,848 proposes to use copolymers obtained by solution polymerization of vinyl esters, while JP-A-59-202463 proposes to use copolymers obtained by solution polymerization of ethylenic unsaturated methacrylates having reactive groups such as phthalimido.
Although the timing layers disclosed in these patents make it possible to compensate the temperature in the neutralization reaction of developing solutions, the range of the temperature thus compensated is very narrow, compared with the temperature range employed in the instant photography in practice, in each case and thus only unsatisfactory images can be obtained thereby.
In addition, the above-mentioned timing layers frequently suffer from a problem that, when stored under high humidity at high temperature, the neutralization timing period is prolonged due to the poor storage stability with time and thus the maximum and minimum densities are elevated. This problem relates closely to the Tg of the polymer material employed in the timing layer. Namely, a higher Tg brings about the more serious problem.
Such a timing layer is frequently located next to the layer wherein an alkali processing composition is developed. However, it is often observed that the timing layer adheres to the opposite layer, between which the development layer is sandwiched, before the development of the processing composition and thus the processing composition cannot be uniformly developed, i.e., an adhesion trouble. This trouble also relates closely to the Tg of the polymer material employed in the timing layer. In this case, the problem is that a lower Tg would cause the trouble more often.
Accordingly, a Tg of the polymer affects to the storage stability with time and to the adhesion trouble oppositely each other, which makes it very difficult to regulate the Tg of a single polymer so as to satisfy both of these requirements.